The @MachineTbl has two "unreleased" models—their 'speedbump' versions of the Q800 and the Q650. The clock chipping guide has two unknown/unused Gestalt IDs! THAT'S WHAT THE UNUSED GESTALT IDs ARE FOR! These 'SpeedBump' models! Each of them!
OKAY elaboration time.
So, inside of
this module relative to the Macintosh Universal ROM (starting at line 809), there are a number of tables identifying aspects of the computer which (how I understand it) are communicated to the djMEMC on Wombat boards (I'm going to skip over the items relevant to the WLCD and Cyclone / Tempest boards) and determine a number of important component timing figures. Several tables that are associated with several properties (I'm sorry if you're a programmer and I'm butchering nomenclature here):
@djConfigTable
Code:
@djConfigTable
@dj20Config dc.w %0000000100000001 ; fastwr=1, ROMspeed=1
@dj25Config dc.w %0000000000011010 ; drpchg=1, drpw=1, ROMspeed=2
@dj33Config dc.w %0000000010100011 ; mhz33=1, cyc23ta=1, ROMspeed=3
@dj40Config dc.w %0000001011110100 ; dwcpw=1, mhz33=1, drcpw=1, cyc2ta=1, drpchg=1, ROMspeed=4
@djRefreshTable
Code:
@djRefreshTable
@dj20Refresh dc.w 285 ; (20MHz * 15.6µs) - 27
@dj25Refresh dc.w 363 ; (25MHz * 15.6µs) - 27
@dj33Refresh dc.w 487 ; (33MHz * 15.6µs) - 27
@dj40Refresh dc.w 597 ; (40MHz * 15.6µs) - 27
@MachineTbl
Code:
@MachineTbl ; Type CPU VIA ID
dc.b @DJ_ORIG,%00010010 ; 33MHz Frigidaire package (Quadra 800)
dc.b @DJ_BUMP,%00010110 ; 40MHz Frigidaire package (unreleased)
[redacted for clarity]
dc.b @DJ_ORIG,%01000110 ; 25MHz Lego package (Centris 650)
dc.b @DJ_BUMP,%01010010 ; 33MHz Lego package (Quadra 650)
dc.b @DJ_BUMP,%01010110 ; 40MHz Lego package (Quadra 650 SpeedBump, unreleased) <SM60>
dc.b @DJ_ORIG,0 ; if here, this is EndOfTable
; VIA CPUID of 0 is a reserved (unused) machine.
(@BumpConfigTable)
Code:
@BumpConfigTable
@bump20Config dc.w %0000000100000001 ; fastwr=1, ROMspeed=1
@bump25Config dc.w %0000000000011010 ; drpchg=1, drpw=1, ROMspeed=2
@bump33Config dc.w %0000000011111011 ; mhz33=1, drcpw=1, cyc2ta=1, drpchg=1 drpw=1, ROMspeed=4
@bump40Config dc.w %0000001011111100 ; dwcpw=1, mhz33=1, drcpw=1, cyc2ta=1, drpchg=1, drpw=1, ROMspeed=5
So we have a swath of properties that cover the gamut of the Centris/Quadra lineup: planned clock speeds (from the lowly Centris 610 all the way up to the 'Cyclone' 840av), DRAM timings, ROM(?) speed(timing?), and a few other properties I haven't seen elaborated upon. Pay attention to the number of machines in the @MachineTbl.
The Wombat boards came in two flavors, if you will: "Figidaire" (also referred to as "inTheFridge" or "fridge") and then "Lego" based on whichever case they were to end up in. They came in three (shipped) configurations at two clock speeds: 25Mhz and 33MHz. There are also two 'unreleased' configurations, both at a 40Mhz clock. These are
not the 840av/Cyclone, these are
Wombat configurations.
My first hypothesis is that these make up the five Gestalt IDs from the archived clock chipping guide. The ones that don't work with System 7.5 without an enabler patch are these 'speedbump' 'bump' machines:
In cases where someone chips the CPU to operate at a higher-than-stock speed, they have two changes to make. If it's a "@DJ_ORIG,%01000110" machine (i.e. Centris 650), they cannot go beyond 30 Mhz, and must switch the board's Gestalt to either a Quadra 650 or 800. If it's already a Quadra, there's no need for that change, however, the second change is altering the on-board clock oscillator. As far as an 'overclock' is concerned, that's the job done. We have ample accounts of folks chipping their quadras to the supposed limit of 44 Mhz, even with a comparatively faster CPU installed.
Here's where a second hypothesis comes into play.
When you have or change to the Gestalt IDs 35 or 36, the ROM is communicating to the djMEMC (memory controller) to expect a machine running with a CPU clock of 33(.333)MHz, and will adjust the relative board component timings accordingly to
that set of figures from ROM,
not to the relative clock you have set via the oscillator. Therefore, you're not overclocking it from the timings of a 40Mhz machine, you're overclocking it from the timings from a 33MHz machine!
So here we see that attempting to reach 50 MHz in this situation, you're putting tremendous strain on the timings and opening up a chasm for errata and system crashes. This is the reason, I hold, that these overclocks haven't been possible for Wombat Macs.
That last column, in particular, is the key point: If hypothesis 1 is correct, choosing a Gestalt ID of 51 or 59 will essentially tell the djMEMC to set the timings for
an actual 40MHz Mac and chipping said Mac to 50Mhz will theoretically only push a DRAM timing overclock of 126%—well within the boundaries of what has been proven by solutions like OutputEnablers or MacClipJr on other Centris and Quadra Macs: